Method of treating steel

ABSTRACT

The toughness of a martensitic, air-hardening steel is increased by a novel combination of steps practiced in the course of hot working a billet of the steel. After a partial reduction of the billet the billet is reheated to a temperature at which carbides are dissolved, whereupon the reheated billet is worked to final dimension under conditions to bring about a complete recrystallization in the workpiece, after which the reduced workpiece is cooled so rapidly that precipitation of carbides in the austenite grain boundaries is substantially avoided.

United States atent [72] Inventors Arne Haraldsson Ornsen;

Bertil Ring, both of Hagtors, Sweden [21] Appl. No. 837,766 [22] Filed June 30, 1969 [45] Patented Oct. 26, 1971 [73] Assignee Uddeholms Aktiebolag Hagfors, Sweden [54] METHOD OF TREATING STEEL 10 Claims, 4 Drawing Figs.

[5 2] US. Cl l [50] Field of Search [56] References Cited UNITED STATES PATENTS 2,893,902 7/1959 Roberts et al l48/l2.4 C2ld 7/14 148/12, 12.3, 12.4; 75/126 Primary Examiner-L. Dewayne Rutledge Assistant Examiner-W. W. Stallard Attorney-Picrce, Scheffler & Parker ABSTRACT: The toughness of a martensitic, air-hardening steel is increased by a novel combination of steps practiced in the course of hot working a billet of the steel. After a partial reduction of the billet the billet is reheated to a temperature at which carbides are dissolved, whereupon the reheated billet is worked to final dimension under conditions to bring about a complete recrystallization in the workpiece, after which the reduced workpiece is cooled so rapidly that precipitation of carbides in the austenite grain boundaries is substantially avoided.

METHOD OF TREATING STEEL Balance iron and impurities.

Steels of this kind are used especially for tools-hot work tools-but also for ultra high strength constructions e.g. in aircraft engineering. Short service lives of hot work tools are often caused by an insufficient toughness of the tool material, which normally is subjected to considerable strains in combination with shocks and high frequent temperature altemations. Also for the ultra high strength structural steels the toughness has a dominating importance for the fitness of the materials.

The toughness is especially insufiicient in large-sized steel pieces, which means round bars with diameters more than about 100 mm. and plate bars with the corresponding cross section area. In order to avoid cracking in these heavy bars after hot working one has had to cool the material slowly in an insulating medium (burying in ashes) which, however, has caused carbides to precipitate in austenite grain boundaries which has impaired the toughness of the steel. it has therefore been suggested-US. Pat. No. 2,893,902-that the steel after hot working but prior to a final soft annealing operation shall be reheated-homogenizedto about l,l C. and kept at this temperature for 2-4 hours and thereupon buried in ashes and allowed to cool slowly. As an alternative to burying in ashes there is suggested fast furnace cooling. Suitably also a normalizing operation is carried out prior to the final soft annealing in order to eliminate hard spots and to produce ultra high strength.

It is an object of the present invention to produce a satisfying toughness in workpieces of the above-mentioned steel also in heavy dimensions, by a suitable carbide distribution and a more homogeneous microstructure over the entire cross section of the material.

Another object is to produce the desired structure in a manner which does not involve long cooling procedures but which as a characteristic feature instead comprises a rapid cooling operation.

A third object is to bring about the desired structure without any special homogenization treatment after final hot working, whereby scaling essentially can be omitted.

The method according to the invention herein is characterized by the novel combination of procedure steps recited in claim 1. After a partial reduction of the billet the billet is reheated to a temperature at which carbides are dissolved, whereupon the reheated billet is worked to final dimension under conditions to bring about a complete recrystallization in the workpiece, after which the reduced workpiece is cooled so rapidly that precipitation of carbides in the austenite grain boundaries is substantially avoided. Further features and objects of the invention will be apparent from the following example, which will be described with reference to the accompanying figures of drawing which show the structures of the test samples enlarged 1,000 times. The samples and the figures have corresponding numbers.

EXAMPLE The test materials had the following compositions.

Charge C Si Mn Cr Ni Mo W V E 20697 0.37 0.98 0.44 5.! 0.l7 L30 0.8 0.95

weight-i:

weight-l:

The experiments were carried out in the following manner. The hot rolling of charge E 23708 was interrupted at the billet dimension 200x235 mm. and a billet about 1.5 m. long was cut off. This billet was reheated to 1,1 30 C. at which temperature all carbides are dissolved. After 3 hours-this time may be shortened-the billet was taken out from the heating furnace and was immediately rolled in one single bonded from 200 mm. to mm. with free widening; Le. a deformation powerful enough to cause a complete recrystallization. The billet was cut into three pieces, from which two pieces (samples No. 2 and 3) were allowed to cool free in air to room temperature. After about 48 hours one of them (sample No. 3) was normalized and soft annealed, while the other one (sample No. 2) only was soft annealed. The third part (sample No. 4) was buried in ashes after the final rolling. After about 2 weeks it had reached room temperature and was then nonnalized and soft annealed. Sample No. l emanates from an earlier examination series, charge E 20697. lts treatment consisted in direct rolling to final dimension 530x266 mm., whereupon the billet was buried in ashes and cooled to room temperature and thereupon sofl annealed. All samples were taken from the central portions of the billets. The different treatment steps are below put together in the following scheme.

Steps of treatment Sample No. Charge a-c d e f l E 20697 Hot working Cooling insu- Soft direct to final lated in ashes annealdimension to morn ing 530 X 255 mm; temp.

2 E 23708 Hot working to Cooling in air Soft 200 X 235 mm; to room annealreheating at temp. ing 1130 C for 3h; M 3 E 23708 rolling to final Cooling in air Normal- Soft dimension to room izing anneall55 X 200 mm temp. ing

4 E 23708 Cooling insu- Normal- Soft lated in ashes izing annealto room ing temp.

in all the four cases the soft annealing was carried out by heating to about 840860 C. followed by 10 C ./h cooling to 650 C., thereafter cooling in air. The normalizing treatment-in the cases it occurredinvolved heating to about 950 C., at which temperature the detail was kept for 30 minutes after having been completely heated through, thereupon cooling in air. The slowly cooling treatment, slowly cooling in insulating ashes, required a time of 14 days.

Sample No. 1, located which was taken from a billet treated in a conventional manner, evidenced substantial carbide precipitations in the grain boundaries, FIG. 1.

The structure of sample No. 2, which was cooled in air and thereafter only soft annealed, is somewhat heterogeneous. A continuous network of carbides can be discerned in inherited austenite grain boundaries. The normalizing treatment prior to soft annealing, sample No. 3, has an obvious positive effect towards an homogenous carbide distribution, FIG. 3. By reheating before working to final dimension in combination with air cooling and normalizing one has also obtained a material almost completely free from carbides in the grain boundaries.

The dominating importance of the cooling rate for avoiding carbides in the grain boundaries is apparent from FIG. 4, sample No. 4, which shows that considerable carbide precipitations remain in the grain boundaries. This billet, immediately equipment. rolling, was buried and cooled in ashes and thereafter normalized and soft annealed.

We claim:

1. A method of improving the toughness by producing more suitable carbide distribution and a more homogenous microstructure over the entire cross section of martensitic air hardening steels particularly steels of the type intended for hot working and/or ultra high strength constructions, which comprises the following sequential steps:

a. interrupting the hot working before the billet has achieved final dimension; thereupon b. reheating the billet to a temperature at which all carbides are dissolved;

c. working the reheated billet to final dimension under so heavy an area reduction that a complete recrystallization treatment is carried out; thereupon d. cooling the recrystallized material so rapidly that the precipitation of carbides 70 the austenite grain boundaries essentially is avoided; and

e. soft annealing it.

2. A method according to claim 1 wherein the material is normalized after the cooling but prior to the soft annealing operation.

3. A method according to claim 1 in which the hot working is interrupted when -50 percent remains of the total area reduction, said remaining reduction being carried no after said reheating operation.

4. A method according to claim 1 in which the reheating of the material is effected at a temperature of l,050-l ,200 C.,

preferably to 1,1 00-l l 50 C., during a time of l/2-5 hours.

5. A method according to claim 1 in which the recrystallized material is cooled in air.

6. The method according to claim 2 in which the material is normalized at a temperature of 900-l ,000 C.

7. A method according to claim 1 in which the material is soft annealed by heating to 820-880 C.. followed by slowly cooling to about 650 C., thereafter cooling in air.

8. A method according to claim 1, in which the steel has the composition (in weight percentages):

max. 12 Mo max. 4 V

max. 1.5 Si

max. 2 Mn Balance iron and im urities. 9. A method accor mg to claim 8 in which the steel has the composition (in weight percentages):

Balance iron and impurities.

10. A method according to claim 9 in which the steel has the nominal composition (in weight percentages):

Balance iron and impurities.

* 1i i i i 6 2? UNi'liiD STATES m'rm': 092103 CER'IJLFICATE OF CGRRE TLON Patent No. 3, 61.5, 905 Dated October 26 .1971

Inventor(s) ARNE HARALDSSON OMSEN and BERTIL RING t is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 1 (CU-line 2 in should appear instead of--"70" Claim 3, line 3, "no" should be o ut Sign'ed and sealed this 28th day of March 1972.

(SEAL) Attest: EDWARD. M.FLETCHER, JR. r 7 ROBERT GOTTSCHALK Attesting Officer I C'ommissipner of Patents 

2. A method according to claim 1 wherein the material is normalized after the cooling but prior to the soft annealing operation.
 3. A method according to claim 1 in which the hot working is interrupted when 10-50 percent remains of the total area reduction, said remaining reduction being carried out after said reheating operation.
 4. A method according to claim 1 in which the reheating of the material is effected at a temperature of 1,050*-1,200* C., preferably to 1,100*-1,150* C., during a time of 1/2 -5 hours.
 5. A method according to claim 1 in which the recrystallized material is cooled in air.
 6. The method according to claim 2 in which the material is normalized at a temperature of 900*-1,000* C.
 7. A method according to claim 1 in which the material is soft annealed by heating to 820*-880* C., followed by slowly cooling to about 650* C., thereafter cooling in air.
 8. A method according to claim 1, in which the steel has the composition (in weight percentages): 0.2-2 % C 1-15 % Cr max. 12 % Mo max. 4 % V max. 1.5 % Si max. 2 % Mn 0-15 % Co 0-25 % W 0-0.5 % S Balance iron and impurities.
 9. A method according to claim 8 in which the steel has the composition (in weight percentages): 0.2-0.5 % C 3.0-6.0 % Cr 1.0-3.0 % Mo 0.7-1.5 % V 0.5-1.2 % Si 0.2-0.7 % Mn Balance iron and impurities.
 10. A method according to claim 9 in which the steel has the nominal composition (in weight percentages): 0.37 % C 5.4 % Cr 1.4 % Mo 1.0 % V 1.0 % Si 0.4 % Mn Balance iron and impurities. 